The present invention relates to the detection of symbols by a receiver.
It relates, more particularly, to the detection of symbols transmitted by a transmitter in the form of a succession of pulses.
The detection of a pulse within a noisy signal has always been problematic and was already an issue in the field of radar systems.
In this respect, the CFAR (Constant False Alarm Rate) detector may be mentioned. It consists in determining the maximum number of triggers on noise peaks (false alarms) that the system can tolerate while still meeting the expected performance criteria. While only the noise is present, the receiver executes a calibration phase during which it determines, by dichotomy, the threshold corresponding to this maximum number of false alarms. The threshold value thus obtained corresponds to the best possible sensitivity of detection for a fixed error rate.
The CFAR detector method yields useful results, but it suffers from certain drawbacks directly associated with its principle of operation.
First of all, it exhibits a minimum error rate. Indeed, the receiver positions its threshold in order to obtain the requested number of false alarms, hence of errors, whatever the link conditions. As a result, the receiver is incapable of exploiting very good conditions, a fact which limits its performance. Secondly, the false alarms represent the vast majority of the errors. This imbalance must be compensated by an error correcting coding system. Lastly, the calibration of the threshold can be complex. It may for example be necessary to disconnect the antenna from the receiver in order to ensure the absence of any signal during its calibration. The calibration of the threshold can therefore only be carried out infrequently and outside of any communication phase.
Another known method for detecting pulses consists in looking for a fixed number of triggers on the useful signal. This particularly simple method relies on the knowledge of the number of pulses transmitted per interval of time. The detection threshold is firstly fixed at a high value, then it is lowered by a given value at each iteration, until it allows the expected number of pulses to be detected. Thus, if a transmitted signal comprises eight pulses per symbol-time, the goal of the system will be to obtain eight triggers per symbol-time.
This method is relatively inefficient owing to the fact that the number of pulses per interval of time is not always known in advance by the receiver and may also vary as a function of the link conditions and of the radio channel (number of echoes for example).
Furthermore, the problem of the detection of pulses also arises in systems using the recent UWB (Ultra-Wide-Band) radio technology.
This emerging technology does not use a continuous carrier frequency. Instead of modulating a carrier signal, the information to be transmitted is transmitted directly using pulses of very short duration (between a few hundreds of picoseconds and a few nanoseconds) and hence of very large bandwidth (several GHz). Since the energy—relatively low—of these pulses is spread out over this whole band, the spectral energy density of the signal is very low.
Thus, a UWB pulse signal is not a continuous signal, but a series of very short pulses with a very low duty cycle.
Multiple access is frequently achieved using time hops (Time Hopping) controlled by a pseudo-random sequence. The information can be modulated by varying the amplitude, the shape or even the delay of the pulses. As regards the hopping sequence, this constitutes a characteristic, or “signature”, of the transmitter.
Each pulse must subsequently be detected by the receiver, which may be carried out either by a synchronous detection (coherent or by correlation), or by a quadratic detection. But, in any case, the result of the detection must then be examined in order to decide, depending on the amplitude of the signal or on the correlation level obtained, on the absence or on the presence of a pulse.
The detection of a succession of pulses can then allow the value of a corresponding received symbol to be decided.
The first UWB receivers used relied on a principle for detecting the pulses by synchronous correlation. In these receivers, it is the correlation level obtained that is compared with a threshold level in order to make a decision. Owing to the synchronous nature of these receivers, the correct adjustment of the threshold is, relatively speaking, not too critical in this case. In contrast, the detection of the pulses is much more problematic in the case of the new non-coherent receivers, based on the detection of energy or of amplitude.
When the pulse detection performance is very poor, it cannot then be hoped to detect efficiently one or more symbols transmitted in the form of a succession of such pulses.
For example, when it is desired to detect the presence or the absence of a pulse within a given interval of time, by comparison of the energy or amplitude measured over this interval of time with a detection threshold, the performances of the detection and of the consequent decision making on the value of the transmitted symbol largely depends on the positioning of this threshold. However, an appropriate value for this threshold is not necessarily readily chosen, especially as the optimum value may vary as a function of the radio conditions in particular.
One object of the present invention is to obtain an efficient symbol detection.
Another object of the invention is to obtain an efficient symbol detection, even when the detection of the successive pulses forming these symbols is relatively inefficient. In particular, the symbol detection should be efficient even when the detection threshold used for detecting the pulses is temporarily or more permanently maladapted.
Another object of the invention is to allow symbol detection in a system of the UWB type and, more particularly, by means of a non-coherent receiver.